The rotor support structures for some rotating machines are structurally sized to withstand the loads that may occur following a postulated rotor imbalance event. For example, the rotor support structure for many aircraft gas turbine engines is sized to withstand a postulated blade loss event. Such an event, as may be appreciated, would result in relatively high imbalance loads. Designing the engine to withstand such an event can result in an undesirably heavy engine, which in turn can result in undesirable fuel burn.
Presently, rotor support structures are designed to withstand the loads associated with a postulated rotor imbalance event in a number of ways. For example, the rotor support structure may be designed with sufficient structural capacity to withstand worst case rotor imbalance loads. This solution can result in an undesirably heavy machine. Alternatively, machine weight can be controlled by limiting the imbalance load that may be supplied to the support structure. This can be accomplished through the use of a frangible support section that fails at a predetermined load, or the use of a buckling section that collapses at a predetermined load. Another solution is to control the load that may be supplied to the support structure through the use of limited stiffness within the load path to the support structure. With this latter solution the imbalanced rotor is allowed to orbit about a point near its center of gravity while maintaining acceptable load in the support structure.
Each of the solutions described above, while certainly acceptable and used, do suffer certain drawbacks. For example, as was already noted, designing the support structure with sufficient structural capacity to withstand worse case rotor imbalance loads can undesirably increase machine weight. The frangible support sections and buckling sections are sacrificial components that must be replaced following a rotor imbalance event. Further, it presents a design challenge to create a frangible feature that will fail at a single occurrence of a load scarcely above maximum normal operational loads while maintaining adequate fatigue strength margin of safety at maximum operational loads. The limited stiffness solution can make it difficult to adequately control rotor centerline position in rotor imbalance, maneuver, or bird strike events within the intended operational envelope.
Hence, there is a need for a system and method for limiting rotor imbalance loads supplied to rotor support structures that does not undesirably increase machine weight and/or does not rely on sacrificial components and/or does not make it difficult to control rotor centerline position following a rotor imbalance event. The present invention addresses at least these needs.